In aircraft applications it is highly desirable to minimize the weight of aircraft components as every pound saved in aircraft weight translates to fuel savings and/or greater payload capacity. With respect to turboprop or turbofan engine components, it is well appreciated that the propulsor blades are the most likely candidate for weight reduction since the weights of other related components, e.g. blade retention means, pitch change mechanisms, hub disks, shafts and bearings, are typically directly dependent upon the magnitude of the blade centrifugal loading borne by these components. The propulsor blades per se, however, can be made lighter in weight so long as the centrifugal pull, bending moments, torsion loads and vibratory loads, imposed upon the blades during operation are effectively transmitted to the blade retention means for distribution to the aforenoted load bearing components.
It is known in the art to produce lighter weight propulsor blades of a built-up construction wherein a blade is formed of outer shell made of lightweight composite material, and an internal load bearing spar which is bonded to the interior surface of the shell and extends from within the shell cavity to terminate beyond the shell in a root end which is adapted to be mounted to a suitable blade retention means. Examples of such composite blades are presented in U.S. Pat. Nos. 3,799,701; 4,784,575 and 4,810,167.
The composite fan blade disclosed in U.S. Pat. No. 3,799,701 is formed of two complementary preformed outer airfoil planform shell halves which are bonded together at their outer tips, leading edges and trailing edges about a solid load bearing spar to form an outer shell encompassing all of the load bearing spar except the root portion thereof which extends beyond the shell. The shell halves are fabricated from a composite material having a relatively high modulus of elasticity, such as boron epoxy, boron-aluminum or carbon, and are bonded along their interior surface to the outer surface of the spar sandwiched therebetween. The solid load bearing spar, which is made of titanium, has an integral dovetail root end which is adapted to mount in a blade retention disc or hub for transmitting load from the blade shell, carried through the spar, to the retention disc or hub.
U.S. Pat. No. 4,784,575 discloses a propulsor blade for a counterrotating unducted propfan system wherein a metallic spar is sandwiched between and bonded to two composite material shell halves which, when disposed about the central spar, form the airfoil shell and define hollow forward and aft airfoil cavities. These airfoil cavities, which are filled with lightweight foam material, result in a reduction in the weight of the blade. The composite shell halves are built-up from layers of graphite, boron or S-glass fibers embedded in an thermosetting epoxy resin matrix. The spar, which is made of a high strength, light weight metal or alloy, such titanium or aluminum, has a integral dovetail root end which is adapted to mount in a blade retention disc or hub for transmitting load from the blade shell, through the spar, to the retention disc or hub. The weight of the blade is further reduced by milling cavities within the spar while maintaining the complete surface area of the spar available as a bonding surface to which the composite shell halves are mounted to establish the structural integrity of the blade.
The composite aircraft propulsor blade disclosed in U.S. Pat. No. 4,810,167 is formed of an outer shell of composite material, specifically an epoxy resin impregnated woven fabric made of graphite fibers, which encompasses a support structure also constructed of composite materials. This inner support structure is built-up from a rigid base plug at its root which defines a cavity for supporting a foam core that extends outwardly from the base plug to form a subassembly which is wrapped in layers of unidirectional graphite fibers bonded together by epoxy resin. The outer plies of these graphite fibers are generally parallel to the longitudinal blade axis and extend around the base plug. The end of the base plug, wrapped with these graphite fibers, is adapted to be mounted within a dovetail slot in the blade retention hub or disc. Metal surfaces are bonded to the outer ply of graphite fibers passing around the base plug to protect the plies from being damaged by the lip surfaces of the hub retention slot. The graphite fibers which encompass the foam core and base plug form a load bearing spar which transfers centrifugal and bending loads from the blade to the retention hub, as well as providing torsional stiffness. The outer composite shell provides surface durability and additional torsional stiffness.
In commonly-assigned, co-pending application Ser. No. 07/255,099, filed Oct. 7, 1988, there is disclosed a lightweight prop-fan propulsor blade which is adapted to be pinned to a hub retention member. As the aforementioned blades, this blade is of a spar and shell type construction. The spar, which forms the main load bearing member of the blade, formed of a core constructed of graphite, Kevlar or fiberglass composite material, the core being received at its base within a root fairing to form a subassembly which is wrapped with multiple layers of graphite fibers bonded at their ends to opposite sides of the core and extending around the bottom of the root fairing. This spar is sandwiched between and bonded to the interior surface of an outer composite shell which encompasses the spar and defines forward and aft cavities which are filled with lightweight foam. The outer shell of the blade is formed by mating shell halves made of fiberglass cloth impregnated with a thermosetting epoxy resin. The root fairing defines a hollow core through which a cylindrical metal bushing extends to provide a pin receiving socket which facilitates mounting the blade by means of a pin to a retention member which is rotatably mounted to the retention hub thereby permitting the pitch of the blade to be changed upon rotation of the retention member.